1. Field of the Invention
This invention relates to the measurement of the transmission of light through a medium, which can be either a gas, liquid, or solid, to determine the absorption coefficient thereof, and more particularly to the method and apparatus for determining the particle concentration in a fluid by use of a single photodetector to detect collimated light beams passed through the fluid along a variable light absorption path length.
2. Description of the Prior Art
It is known that the presence of dissolved substances in a liquid gives rise to the attenuation of a beam of light passing through the liquid and that the attenuation occurs selectively at differing wavelengths which are characteristic of different solutes. Hence, by measuring the attenuation of the light, that is, the optical density of the liquid at specific wavelengths, the concentration of the solutes can be measured, thus providing a method of analyzing the liquid. However, suspended solid matter also gives rise to the attenuation of the light passing through the liquid, but in this case, the attenuation occurs generally over most of the optical spectrum due to the absorption, scattering, or reflection of incident light by the particles of the suspended solids. Therefore, there will be an error introduced in the apparent concentrations of the solutes, by the overall attenuation due to the solid matter suspended in the liquid.
In another prior art example, the particulate content of stack emissions for power plants and similar installations must be limited to acceptable levels. This requires continuous monitoring and frequent checking of the performance of the monitoring apparatus.
Measuring optical transmittance of the smoke, etc. is one of the means available to evaluate the amount of particulate matter being discharged to the atmosphere. Since the amount will depend on the dimensions of the smoke stack, etc. as well as on the density of the particles in the gas, in general it is desired to determine that transmittance over a predetermined distance such as the stack diameter so that appropriate standards can be established and compliance therewith can be determined.
To determine light transmittance, it is necessary to transmit light through the gaseous medium and measure variations in attenuation produced by the particles therein. Variations in the intensity of the light source and in the sensitivity of the light sensor will commonly affect the measurement. Also, accumulation of dirt on optical windows through which the light passes will affect the measurement. In addition, scattering of the transmitted light by smoke particles and the like in the gaseous medium may affect the measuring accuracy.
U.S. Pat. No. 3,976,891 to Parkinson discloses a device for measuring the density of smoke which compensates for the accumulation of smoke particles, soot, or dust particles on the faces of windows which are exposed to the air or gas in which the particles to be detected are contained. Two separate windows are provided through which two separate beams of light are passed. The windows are spaced such that the respective beams passing through the windows pass different distances through the smoke. Photoelectric devices receive the light transmitted through the windows, and a bridge circuit compares the respective responses of the photoelectric means to determine the density of the smoke particles.
U.S. Pat. No. 4,017,193 to Loiterman discloses a device to measure the transmittance of a gaseous medium carrying particulate matter which is substantially unaffected by variations in light source brightness, dirt build up on optical windows, scattered light and photosensor sensitivity. First and second light sources and first and second light sensors are positioned opposite to each other. The sources and sensors are positioned to provide equal shorter path lengths between the first source and the first sensor and between the second source and the second sensor, and equal longer path lengths between the first source and the second sensor and between the second source and the first sensor. Each light source produces a narrow collimated beam directed toward each respective light sensor. Output signals of the sensors are fed to computing means which produces a quotient of the products of the output corresponding to light transmitted over the longer and shorter paths in order to measure the density of the smoke.
U.S. Pat. No. 4,037,973 to Carr discloses a sensing unit for suspended solids measurements having a single light source, a pair of photodetectors disposed on a common light path at different distances with respect to the light source, and a measuring circuit operatively connected thereto. The liquid with the suspended solids to be measured is disposed between the light source and the photodetectors, which are supported in fixed relationship with each other. A tubular opaque shield contains the light source and detectors and the ends provide ingress and egress of the liquid.
U.S. Pat. No. 3,652,850 to Briggs discloses method and apparatus for measuring the optical density of a fluid for light having wavelengths in a specific region of the electromagnetic spectrum. A first and second detecting means are provided for receiving light that has traveled through a sample of fluid over both longer and shorter light paths. A means for alternately directing the light through the fluid to associated detecting means so that each detecting means alternately receive light which has traveled along longer and shorter paths. The detectors provide a first signal related to attenuation of light and a second signal related to the concentration of solid matter in the fluid. The second signal is used by monitoring apparatus to correct the first signal for attenuation of the light by the suspended solid matter and thereby to provide an output signal indicative of the density of the fluid.
Particle concentration measuring devices exemplified above generally consist of a light source and a light intensity detector such as a photodiode with a fluid specimen placed between them. In such systems, various sources of errors exist which require expensive compensation techniques; viz., light source intensity variations, stray ambient light and photodiode dark current, detector sensitivity variations, and variations in optical component transmissibility. For example, compensation for light source intensity variations may be accomplished by deflecting part of the outgoing light sources beam with a beam splitter and measuring the intensity of this deflected beam, and compensation for stray light ambient light and photodiode dark current may be accomplished by modulation of the light intensity and use of a lock-in amplifier.
By use of the method and apparatus of the present invention with its unique feature of the use of a variable light absorption path length to measure absorption coefficients, the above errors of the prior art devices are removed.